1. Field of the Invention
The present invention relates generally to the production of driver fluid for a gas turbine and particularly to a gas generator for a stationary gas turbine which cooperates with a waste heat steam generator. More specifically, this invention is directed to a method of and apparatus for operating a gas turbine in a combined gas and steam generation facility and especially to the production of hot, pure gas turbine driver gases from coal. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
In the interest of the use of readily available coal as a fuel, while at the same time minimizing the potential for environmental damage, it has been proposed that pulverized coal be burned and the hot exhaust gases resulting from the combustion be employed as the driver gas for a turbine in a combined gas and steam turbine generating facility. In such a facility, fine-grained coal or coal dust is mixed with a source of oxygen, i.e., air, air enriched with oxygen or pure oxygen, and burned at a pressure in excess of one bar and at a temperature in excess of 1,000.degree. C. The combustion process may be enhanced by recirculation of the exhaust gases through the combustion chamber, such recirculation ensuring the appropriate combustion temperature. The combustion process produces an exhaust gas that basically comprises CO.sub.2 and steam. When air is delivered to the combustion chamber, the exhaust gas may also comprise nitrogen. Prior to use of a hot exhaust gas produced through the burning of pulverized coal to drive a turbine, the gas must be cleaned of errosive particulate matter and corrosive material such as alkali metal compounds. Additionally, it is necessary to remove SO.sub.2 and NO.sub.x from the gases resulting from the combustion process. Once purified, the hot exhaust gases may be caused to serially flow through a gas turbine and a waste heat steam generator. In the waste heat steam generator, heat will be exchanged from the exhaust gases to water which is preheated, evaporated and superheated to operate a steam turbine on one or more levels of pressure.
An example of a coal fueled gas and steam generating facility employing a gas turbine may be seen in VGB Kraftwerkstechnik published in 1990, volume 5, pages 399-405. It is essential that the gas resulting from the combustion process in such a facility be purified prior to delivery to the gas turbine, i.e., gases produced through the combustion of coal contain matter and substances, as discussed above, that would damage a gas turbine. The need for purification, however, presents a substantial problem because efficient operation of a modern gas turbine requires that the inlet temperature exceed 1,200.degree. C. However, known and effective gas purification technology requires that the temperature of the gases to be cleansed of pollutants be lowered to a level in the range of 650.degree.-950.degree. C. This relatively low temperature is particularly critical for the dry additive process, i.e., the desulfurization by lime dust injection, and the selective non-catalytic reduction process (SNCR), i.e., reduction of nitrous oxide with ammonia without a catalyst. In the prior art, in order to lower the temperature of the exhaust gases resulting from the combustion of pulverized coal to the 650.degree.-950.degree. C. range, the exhaust gases were passed through a water-steam heat exchanger and/or air was injected. The temperature reduction results in diminished plant efficiency, as mentioned above, due to the lowering of the gas turbine inlet temperature.
The desire to minimize the possibility of environmental damage, and governmental regulations which seek to achieve this objective, demand that machinery such as gas turbines be operated with maximum efficiency. Obviously, the more efficiently the facility operates, the less fuel which must be burned to achieve a given output.